Crystal triodes



March 27, 1956 K. A. MATTHEWS ETAL 2,740,076

CRYSTAL TRIODES Filed April 4, 1952 F/ G. /\/\/l 5aw- T ot/1 3 2 WaveGener t l4 1 l #--/s O Vo/ts 7 Inventor K.A.MATTHEWS- W 9 By AttorneUnited States. Patent CRYSTAL TRIODES Kenneth Albert Matthews andCharles de Boismaison White, London, England, assignors to InternationalStandard Electric Corporation, New York, N. Y.

Application April 4, 1952, Serial No. 280,692 Claims. (Cl. 317-435) Thepresent invention relates to electric amplifying devices employingsemi-conductors which have been called for convenience crystal triodes,this application being a continuation-in-part of our co-pendingapplication 150,- 412, filed March 18, 1950, and entitled ElectricSemiconductor, now Patent No. 2,653,374.

For the purpose of this specification, a crystal triode means. a body ofsemi-conducting material (such as a crystal of germanium) having inrectifying contact with its surface at least two electrodes placed closetogether, but not in actual contact. One of these electrodes is calledemitter electrode and the other is called the collector electrode. incontact with the semi-conducting body is also a third electrode calledthe base electrode which may take the form of a metal cup or holder onwhich the semi-conductor is mounted, although the use of the term base:electrode should not be taken as restricting the invention to suchamplifying devices in which the third electrode is of extended area.

The emitter and collector electrodes of a crystal triode may consist offine Wires or cat whiskers. The emitter electrode may be used as aninput electrode of the crystal triode, and the collector electrode maybe used as an output electrode.

By means of a suitable associated circuit, a power gain may usually beobtained with a crystal triode but sometimes it also produces a currentgain or develops current gain after an overload in normal use, and thisis liable to produce unstable conditions in certain applications of thedevice, and may, therefore be very undesirable. Conversely, while acurrent gain is desirable in many applications, it has heretofore notalways been possible to obtain a current gain with a crystal triode.

The principal object of the present invention, therefore, is to provideimproved crystal triodes of two types, one type being capable of givinga large current gain and. the other type being capable of giving poweror voltage gain, without producing an accompanying unwanted currentgain.

A crystal triode is said to give a current gain when a given change inthe emitter current produces a larger change in the collector current,on the assumption that the output load impedance connected to thecollector electrode is zero.

In the study of semi-conducting materials for use as rectifiers, it hasbeen the practice in some circles to divide the materials into twoclasses, namely N-type materials and P-type materials. In the N-type,the conduction of the current in the material is principally due to themigration of a few free electrons, While in the P-type, it is said to bedue to the migration of what are called positive holes, that is,deficiencies of electrons in a few atoms of the material.

Up to the present, crystal triodes have been generally constructed withN-type material, and germanium has been'commoiily used as thesemiconductor. In order to operate as an amplifier, it is necessary totreat the surface by known methods, to produce good rectifyingproperties.

These methods preferably involve grinding or polishing the surface ofthe semiconductor, and then etching the polished surface with a solutioncontaining nitric acid, hydrofluoric acid, and copper nitrate. Therectifying properties result from the formation by this means, on thesurface of the semiconductor, of a thin layer having the opposite typeof conductivity to that of the body of the semiconductor, that is, aP-type layer if the semiconductor is of the N-type, or an N-type layerif the semiconductor is of the P-type.

In constructing a. crystal triode from a semiconductor treated as justdescribed, the two cat whisker electrodes already mentioned are arrangedin rectifying contact with the treated surface. Then the emitter orinput electrode has to be polarised positively with respect to thecollector or output electrode when the main body of the semiconductor isof the N-type. It is however also possible to produce a crystal triodefrom a semiconductor of the P-type (which, as already explained, willhave an N-type layer on the surface after treatment), in which case theemitter electrode must be polarised negatively to the collectorelectrode.

A particular electroforming process is described below, by means ofwhich the crystal triode may be caused to produce a current gain. Thisprocess is also described and claimed in the specification of the aboveidentified application Serial No. 150,412. It is pointed out in thatspecification that to produce a current gain by mcans'of theelectroforming process described, it is necessary that the collectorelectrode should contain an additive material which acts as a donor oracceptor impurity for the semiconductor when the latter is of the N- orP-type, respectively.

It has already been proposed to employ Phosphor bronze (among othermaterials such as tungsten and copper) as the material for the catwhisker electrodes of a crystal triode, and also to employ anelectroforming treatment. Owing to the manner of its production,Phosphor bronze sometimes contains phosphorus as an impurity, but it isnot an essential constituent, and phosphorus is a suitable donormaterial for germanium. However, in the early proposals alreadyreferred. to, the use of a donor impurity was evidently not material,since tungsten, for example, was used interchangeably with Phosphorbronze.

The new advance in the art constituted by the present invention,according to one of its aspects, is the recognition that a crystaltriode can be caused to give a large current gain if an additional thinlayer of the same con-- ductivity type as the main body of thesemiconductor is produced on top of the layer of opposite conductivitytype prociluced by the conventional surface treatment normally use Theadditional layer is preferably obtained by applying. the electro-formingtreatment described below, using, a collector electrode containing a.donor or acceptor impurity according as the main body of thesemiconductor is of the N- or P-type, respectively. The effect of thistreatment is to inject some. of the impurity from the collectorelectrode into the surface of the semiconductor, thus producing thesecond layer of the same conductivity type as the main body of thesemiconductor.

While the electroforming process described below is a convenient andpreferred process for carrying out the operation under propervcontrolled conditions, it is not the only process which could be used.It has, for example,

been discovered that crystal triodes originally having a voltage gainbut no current gain are sometimes. found subsequently to acquire acurrent gain as a result of theapplication of some accidental pulse, ifthe collector electrode contains an additive of the proper type, thesaid pulse driving some of the additive material into the crystal surface. Then the circuit may be found to be unstable. A small amount ofphosphorus, antimony or arsenic in the collector electrode would havethis effect.

As already pointed out, the object of the invention covered byapplication No. 150,412 isto provide a particular electroforming processfor a crystal triode. The object of the present invention, however, isto provide a crystal triode which can be arranged to produce either acurrent gain or a voltage gain.

Another object of the invention, as stated above, is to provide acrystal triode having a voltage gain, which will have no tendency toproduce a current gain. This object is achieved, according to theinvention, by providing the triode with a collector electrode which isfree from an impurity which is of the type to act as a donor impurity inthe case when the semi-conductor is of the N-type, such as germanium,for instance, or as an acceptor impurity when the semi-conductor is ofthe P-type. Thus electrodes of pure tungsten will be satisfactory forthis type of crystal triode, and this is the preferred material forthese electrodes. Pure Phosphor bronze could be used, provided that itis absolutely free from contamination with phosphorus.

Another object is achieved according to the invention by providing acrystal triode in which the surface of the semi-conductor is treated inthe usual manner to obtain good rectifying properties, by producing onthe surface of the semi-conductor a layer of the opposite conductivitytype, and is then provided with a second layer over part of the firstlayer, with which second layer the collector electrode is in contact,such second layer containing as an additive a donor impurity when thesemi-conductor is of the N-type or an acceptor impurity when thesemi-conductor is of the P-type.

According to another aspect, the invention provides a crystal triodecomprising a body of semi-conducting material of a given conductivitytype, having a rectifying surface layer of the opposite conductivitytype, a base electrode in contact with the semi-conducting material, andcollector and emitter electrodes placed close together on the saidsurface layer, and making rectifier contact therewith, at least thecollector electrode containing as an additive material, a donor or anacceptor impurity, according as the semi-conducting material is of theN- or P- conductivity type, respectively.

The invention will be described in relation to a crystal triodecomprising either an N-type or P-type semi-conducting substance incontact with which are the two cat whisker electrodes of small contactarea placed very close together.

In one manner of carrying out the invention when a crystal of N-typegermanium is used as the semi-conducting substance, the material of thecollector electrode contains as an additive a small quantity ofphosphorus, or antimony, or arsenic, or other donor type of impurity.

As already mentioned, Phosphor bronze often (but not always) containsphosphorus as an impurity, and if so, it could be used as the materialfor the collector electrode, and may be used also for the emitterelectrode. This is a very convenient material since it can be so easilyprocured.

Another suitable donor impurity is arsenic, and the collector electrodemay comprise, for example, a sharply pointed tungsten wire on thesurface of which has been deposited a small quantity of arsenic, eitherby evaporation, or by deposition from a solution containing arsenic.Antimony has also been successfully used as a donor impurity. Theemitter electrode could be a sharply pointed tungsten wire with orwithout arsenic on its surface. A suitable electroforming treatment isthen applied for the purpose of'injecting some of the arsenic from thecollec-. tor electrode into the surface of the germanium, in order toform the second layeraiready mentioned. A preferred process will bedescribed below. Previously to electroforming it is necessary that thesurface of the germanium crystal should have been suitably treated inknown manner in order to produce good rectifying properties.

It is necessary to make clear that once the second layer has been formedby using a pair of electrodes, of which the collector electrode has thedonor impurity, and injecting the phosphorus or other donor impurityinto the first layer, the said electrodes could, if desired, be replacedby others of which the collector electrode only is placed in contactwith the second surface layer, such other electrodes not containing anydonor impurity. For example, cat whisker electrodes of pure tungstencould be used in this case, or electrodes plated on in the mannerdescribed in the specification of co-pending application No. 228,486,now Patent No. 2,680,220.

When cat whisker electrodes are used, the pointed ends should preferablybe placed about two or three thousandths of an inch apart, but with theemitter electrode outside the second surface layer. It has been foundthat a chisel point obtained by cutting the wire in a plane making asmall angle with the aXis of wire is satisfactory, though a sharpconical point produced electrolytically can be used.

In making one form of a crystal triode, the germanium is melted and castinto suitable ingots, and a disc of suitable size is cut from an ingot.The usual surface treatment such as is customary to apply to germaniumused in a diode rectifier to produce good rectifying properties, isgiven to the surface to which the point contacts are to be made.

After the germanium surface has been treated and provided with a pair ofcat whiskers of which at least the collector electrode contains eitheran acceptor or a donor impurity, a suitable electroforming processshould be applied, for the purpose of injecting some of the impurityinto the surface of the germanium.

While the principles of the invention have been described above inconnection with specific embodiments and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

A preferred process will be described with reference to the accompanyingdrawing in which:

Fig. 1 shows an electroforming and testing circuit for carrying out theelectroforming process;

Fig. 2 shows the characteristic obtained on the cathode ray tube used inthe testing circuit; and

Fig. 3 is a cross-sectional view greatly enlarged, of a crystal triodeaccording to the present invention.

This, however, is not the only type of process which could be used,according to the invention.

In the forming circuit, Fig. 1, a crystal triode is shown in section andconsists of a disc or plate 1 of germanium secured to a metal base 2having a base terminal lug 3. The emitter electrode is shown at 4 andthe collector electrode at 5.

According to one concept of the invention, at least the collectorelectrode 5 should be constructed of a metal or alloy having a donormaterial as an additive if N-type germanium is used.

A generator 6 of saw tooth waves, having a low impedance output circuitcapable of supplying currents up to about milliamperes, has one terminalconnected to ground and the other connected to the emitter electrode 4.The circuit is completed by two resistances 7 and 8 connecting thecollector electrode 5 to ground, and a switch 9 is provided by means ofwhich the resistance 3 can be short-circuited.

The testing device comprises a cathode ray tube 10 of conventional type,only the deflecting plates of which are shown. The horizontallydeflecting plates 11 are connected respectively to the electrodes 4 and5, and the vertically deflecting plates 12 are connected across the (newresistances land 8. It will thus be clear tfiatthe Kori-1 zontaIdetlection-of the cathode ray will be to -the voltage applied between:the emitter and collector electrodes 4' and 5, and the verticaldeflection; will be proportional to the c'urrent'which passes betweenthe two electrodes.

It should be carefully noted that for properly froming the crystaltriodeformed from N-type material; the generator 6 should. provide a positiveoutput voltage so that the emitter electrode is always positive to thecollector electrode. The saw-tooth waves should preferably be such thatthe output voltage varies steadily from zero to a positive value whichmay be between 40 and 100 volts, and then flies back rapidly to zero. Itis also very important that the base electrode 3 should be leftunconnected.

The resistances 7 and 8 should be chosen so that normally a smallcurrent flows in the forming circuit. It is then found that generallythe curve traced on the oscillograph screen is at first as shown in Fig.2 by the full line curve 13, 14. This curve shows the current in thecircuit as ordinates plotted against the voltage between the emitter andcollector electrodes as abscissae, for the forward or scanning strokesof the saw tooth waves. A different return curve is traced during thefly back strokes, but this is of no interest and is not visible owing tothe rapidity of return of the spot to zero. It will be seen that thecurve 13, 14 has a loop with a portion 14 having a negative slope,indicating a negative resistance condition between the electrodes 4 and5.

In order to form the crystal triode, the switch 9 is momentarily closed,thus short-circuiting the resistance 8 and greatly increasing thecurrent in the circuit. This injects. some of the donor material intothe surface of the crystal.

At the same time, the curve traced on the tube 10 will momentarilycollapse downwards due to the reduction of the vertical scale by theshort-circuiting of the resistance. When the switch is reopened, it willusually be found that the loop of the curve 13 will have become reduced,and in some cases will have completely disappeared, the curve on thescreen then being as shown partly by the full line 13, and partly by thedotted line 15. If the loop does not disappear completely the firsttime, by repeating the process two or three times, the loop can beentirely re moved, so that the characteristic curve follows the smoothdotted portion 15. This is the condition under which it will be foundthat the crystal triode produces the maximum current gain.

It should be particularly noted that the forming process is carried outbetween the two cat whisker electrodes and not between either and thebase 3, which is left unconnected in the forming circuit. Further, theemitter electrode should be positive to the collector electrode duringthe forming.

It should be added that the use of saw-tooth waves is not essential forforming the crystal triode according to this invention; for example,positive rectangular pulses could be used, or the positive loops of sinewaves. However, the use of saw-tooth waves is very convenient when it isdesired to carry out the forming and testing process in the samecircuit.

It is further to be noted that a low reverse resistance measured betweenthe collector electrode and the base 3 (treated as a simple rectifier)is detrimental to good current and power gain characteristics and shouldtherefore be avoided. This is one of the reasons for forming between theemitter and collector electrode, because it is found that if theelectroforming is carried out between the collector electrode 5 and thebase 3, a low reverse resistance sometimes results, particularly whenusing a soft metal for the collector electrode.

It should be pointed out that if the process of forming should beoverdone, the loop with the negative slope may reappear. It is foundthat by repeating the process the loop lie-made to collapse again sothat the smooth curve 15 can always be produced.

A' further point to note is that it may occasionally be found that theforming process fails to reduce the loop. Thereasonfor this isnotclearly understood, but it will generally be-found that the desiredresults can be obtained by shifting the catwhiskers to another part ofcrystal. Very rarely no part of the crystal surface can be foundwhich'is-satis'faetory, and in this case the surface should be groundof! and re-treated, and the electro-forrning repeated when the desiredresults will generally follow.

It should be understood that the crystal triode may take other formsthan that shown in Fig. 1. For example, it may consist of a disc with avery thin central portion, the emitter and collector electrodes makingcontact with opposite side of the thin portion.

It has been assumed so far that the crystal triode em' ploys an N-typesemi-conductor. However, if the semiconductor should be of the P-type,the arrangements described with reference to Fig. 1 will be the same,except that according to the invention, the collector electrode, shouldcontain an additive of the acceptor type, and that the generator 6should be arranged to supply a negative voltage to the emitter electrode4 instead of a positive voltage.

In other words, whichever type of semi-conducting material is used, thepotentials of the emitter and collector electrodes should be such thatthe emitter electrode 4 is biased in the conducting direction whenconsidered as forming a rectifier with the semiconductor.

To sum up, referring to Fig. 3, it may be said that a crystal triodeadapted to produce a current gain consists of a semi-conductor body 16having a surface which has been treated in conventional manner toproduce a surface layer 17 of opposite conductivity type in order toobtain good rectification properties, and on this surface is a secondlayer 18 containing a donor impurity if the semiconductor is of theN-type or an acceptor impurity if it is of the P-type. The collectorelectrode 19 is placed in contact with this second surface layer 18,while the emitter electrode 20 is in contact with the first surfacelayer 17. The usual base 21 is provided.

One way of producing this second surface layer is to provide twoelectrodes making rectifier contact with the treated surface, thecollector electrode at least containing the appropriate impurity, andthen to carry out between the electrodes an electro-forming process suchas that which has been described.

Conversely, a crystal triode adapted to give a voltage gain without anattendant current gain comprises a semiconductor which has been treatedin the same conventional manner with at least the collector electrodebeing of a material which is free from an impurity of the type to act asdonor or acceptor impurity for the semi-conducting material, accordinglyas the latter is of the N-type or P-type respectively.

What we claim is:

1. A crystal triode comprising a main body of semiconducting materialand having on one of its surfaces a first layer of opposite conductivityto the main body, a second layer on said first layer and of oppositeconductivity to said first layer, a base electrode in contact with saidmain body, and two additional electrodes making rectifying contactrespectively with said first and second layers.

2. A crystal triode according to claim 1 in which the said additionalelectrodes are constructed of a Phosphor bronze.

3. A crystal triode according to claim 1 in which the semi-conductingmaterial comprises N-type germanium and in which the additionalelectrodes contain either phosphorus, arsenic or antimony.

4. A crystal triode according to claim 1 in which the semi-conductingmaterial comprises N-type germanium,

7 and in. which said second layer comprises eitherphosphorus, arsenic,or antimony. '1

5. A crystal triode comprising a body of semi conducting material of agiven conductivity type, having on its surface a rectifying first layerof the opposite conductivity type, a second layer arranged over part ofthe first layer and containing as additive material a donor or acceptorimpurity according as the semi-conductingmaterial is of the N-type orthe P-type, respectively, a base eleetrodejn contact with thesemi-conductingmaterial, and two additional electrodes, at least one ofwhich consists of sharply pointed tungsten wire having arsenic or anarsenic compound deposited on its surface to be used as a collector.

I 4 References Cited in the file of this patent 1 UNITED STATES PATENTS2,653,374 Matthews et a1. Sept. 29, 1953 Bardeen et al; Oct. 3', 1950'

1. A CRYSTAL TRIODE COMPRISING A MAIN BODY OF SEMICONDUCTING MATERIALAND HAVING ON ONE OF ITS SURFACES A FIRST LAYER OF OPPOSITE CONDUCTIVELYTO THE MAIN BODY, A SECOND LAYER ON SAID FIRST LAYER AND OF OPPOSITECONDUCTIVITY TO SAID FIRST LAYER, A BASE ELECTRODE IN CONTACT WITH SAIDMAIN BODY, AND TWO ADDITIONAL ELECTRODES MAKING RECTIFYING CONTACTRESPECTIVELY WITH SAID FIRST AND SECOND LAYERS.